Test circuit for curiosity : 2 x NE555 (astable + PWM) + n-mosfet + var 10k + pc fan

[bendeg]

Hi reader,

I'm trying to control a pc fan speed by giving PWM to the gate of a n-mosfet.

1) generation of a clock with a 555 timer (~ 1,5 kHz, as seen in the datasheet, fig.12) (Vcc = 6V).
Output to TRIG of second timer (pin 2) + red led

2) generation of a PWM with a second 555 timer (as seen in the datasheet, fig.18), CONT (pin 5) modulated by a variable 10k resistor (Vcc)

3) Using output of second timer to "pulse" n-mosfet gate (MDP13N50) + yellow led

4) pc fan (12V, 0.18A) in series in drain-source of mosfet (with another 12V power supply)

(see circuit diagram attached, circuit made on a breadboard)

Looks like it works at least partially : I can see the second led (yellow) becoming brighter/dimmer when I change potentiometer value but the fan isn't doing anything...

BUT : fan spins only if I touch the ground of first power supply (Vcc=6V, suppplying both timers) and the drain (= the case) of the mosfet, then and only then, speed change if I change potentiometer.
Sometimes, when I remove my hand from ground, it spins even faster (not much though), sometimes not...

Nothing burns, nothing is hot...

What am I doing wrong ?

[eccerr0r]

Re: Test circuit for curiosity : 2 x NE555 (astable + PWM) + n-mosfet + var 10k + pc

more mosfet abuse:

mosfets are not relays. And you still need a common ground.

mosfets will short drain to source if the voltage from gate to source exceeds its threshold.

So if PS1 and PS2 are galvanically isolated from each other, what you should do is hook up PS1's GND to the source of your mosfet, and it should work.

If PS1 and PS2 are NOT galvanically isolated from each other and share a same ground, you cannot use the circuit as is. The easy solution is to use the mosfet to "switch" the GND of the fan - connect the fan v+ directly to PS2, fan's V- to drain of the mosfet, and mosfet's source to GND.

of course a dim LED does not necessarily mean the fan will behave the same way so likely you'll still need to change the time constants of the second 555.

[bendeg]

Re: Test circuit for curiosity : 2 x NE555 (astable + PWM) + n-mosfet + var 10k + pc

I tried your "easy solution and, indeed, it works.
GNDs of PS1 and PS2 are connected (I tried that as well before posting, without success...).

But now I can hear what I think is most probably the frequency of the PWM : the sound, which is not loud, definitely comes from the fan circuit.
The tone gets higher if I make it faster (10k-pot of 1st timer AND 10k-pot to modulate second timer).
Funny but a bit scary though, I don't like that much, looks like it will break or something if I let it turn too much time...But again, no magic smoke or magic smell...

For the time constant of second 555, I read in the datasheet that it should be close to 1/4 of the first timer constant (clock), (period ~ 14.5 ms).
By using a 220R with a 22nF capacitor, I think I'm not that far from that ratio (~4.8 ms, a bit too long though), maybe i should chose a lower resistor.

Anyway, thank you very much for your advices ! Undoubtely , I've more to learn...

[momaka]

Re: Test circuit for curiosity : 2 x NE555 (astable + PWM) + n-mosfet + var 10k + pc

If all the O/P is trying to do is to control the fan speed with a PWM signal, this can easily be done with only ONE 555 timer and N-ch MOSFET...

If tachometer output from the fan (assuming a 3-wire PC fan) is to be preserved and accurate, then replace the N-ch MOSFET with a PNP BJT (and switch its position to modulate Vcc to the fan rather than modulate its ground). If driving a bigger PNP, might have to use an NPN driver circuit with a collector pull-up resistor for the NPN (base "pull-down" resistor for the PNP.)

[bendeg]

Re: Test circuit for curiosity : 2 x NE555 (astable + PWM) + n-mosfet + var 10k + pc

@momaka,

Indeed, it's possible to modulate duty cycle with only one 555. I'll try that.

I'll try with BJTs once I've got a bigger transistor than a tiny BC547, max 100mA for collector current if I'm not mistaken (fan demands twice as much). Or use 2 of them in parallel, is that possible to do ?

But, as I can see on the net, those 3 pins fans aren't very well suited for PWM driving, better use 4 pins fans which are designed to be controlled with PWM...

Anyway, thank you very much for your advices !

[bendeg]

Re: Test circuit for curiosity : 2 x NE555 (astable + PWM) + n-mosfet + var 10k + pc

Little update for those who might be interested.

Different tests :

1) Same config as original but n-mosfet changed : IRF520 (2V < Vgs < 4V), instead of a MDP13N50 (3V < Vgs < 5V)
=> works better.

2) Without second 555 and Ra = 10k-pot instead of 1k-fixed to modulate duty cycle) : output of 555 direct to base of IRF520.
Working.

3) Same config as point 2) but replacing n-mosfet IRF520 with 3 npn BC547 in parallel (fan demands ~200mA, BC547 collector-emitter supports only 100mA current) and a base 10k limiting resistor. No limiting resistor in collector-emitter circuit. Fan on collector side.
Working.


There's no annoying audible noise if frequency if high enough : tested from ~48kHz => ~230kHz (measured by my DMM, I wish I had an oscilloscope...).

It would probably better to use other pot values for Ra and Rb : speed variation is not linear (in all cases), I think max fan speed isn't reached.
It is rather difficult to tune the speed smoothly in a linear way.